The present invention relates to weatherable and abrasion resistant coating systems for polymeric substrates.
Polymeric materials, such as polycarbonate, are promising alternatives to glass for use as structural material in a variety of applications, including automotive, transportation and architectural glazing applications, where increased design freedom, weight savings, and improved safety features are in high demand. Plain polycarbonate substrates, however, are limited by their lack of abrasion, chemical, UV and weather resistance, and therefore need to be protected with optically transparent coatings that alleviate above limitations in the aforementioned applications.
To impart abrasion resistance to the polymeric materials, polycarbonate substrates are in general coated with thermally curable silicone hardcoat. The poor weatherability of polycarbonate, on the other hand, is addressed with addition of organic or inorganic UV-absorbing materials in the silicone hardcoat layer. However, incorporation of UV absorbers, especially organic based, in the thermal curable silicone layer, often leads to inferior abrasion resistance performance.
One approach to address the limited abrasion resistance performance associated with the use of organic UV-absorbing materials is to use inorganic UV-absorbing materials at least partially in lieu of organic absorbing materials. The expected benefit is to avoid the addition of large amount of organics in the silicone hardcoat, thereby keeping abrasion resistance characteristics intact. In addition, given the photo and oxidative stability of inorganic UV-absorbing materials compared with organic UV absorbers, the use of inorganic UV-absorbing materials can potentially help to achieve weatherability for extended period of time.
However, it is technically challenging to incorporate inorganic UV-absorbing materials in the form of colloidal dispersion into an organic based coating composition, either with or without the presence of colloidal silica in the coating composition. The challenges relate to the ability to obtain long-term stable inorganic UV absorber dispersions, the ability to inhibit the agglomeration of colloidal particles of the inorganic UV absorbers, and the ability to maximize the loading of inorganic UV absorbers in order to maintain abrasion resistance characteristics without adversely affecting the film uniformity.
Another approach to address the limited abrasion resistance of thermal curable silicone hardcoat, is to deposit an abrasion resistant coating such as a hydrogenated silicon oxycarbide layer on top of commercially available thermally cured hardcoat systems by plasma techniques. However, although the outdoor weatherability for such a system might be slightly improved compared with commercially available coating systems, the hydrogenated silicon oxycarbide layer-containing system is still not able to meet the 10-15 year durability requirements that the market demands. Further, such a coating system is a relatively complex multilayer structure that is difficult to manufacture. Due to the costs of the raw material and the costs of processing and integration of multiple layers, the polycarbonate substrate coated with a hydrogenated silicon oxycarbide comprising system can be much more expensive than conventional glass.
In order to address the manufacturing complexity of hydrogenated silicon oxycarbide comprising systems, improvements have been disclosed for example in patent application publication US 20060204746, where an inorganic weatherable film layer is adhered to the polymeric substrate and a plasma deposited hydrogenated silicon oxycarbide abrasion resistant layer is directly deposited on the inorganic weatherable film layer. However, to insure adhesion between the weatherable film and the substrate, as well as the weatherable film and the abrasion resistant layer, one or more adhesion promoting interlayers are typically required, thus complicating this manufacturing process as well.
Hence, there is a continuing need for a protective coating method and composition for polymeric substrates that is effective to provide AS-2 type abrasion resistance, long-term outdoor weatherability at a structure that is easier to manufacture than is currently available in the art to the knowledge of the present inventors. The present invention provides one answer to that need.